The present invention relates to lithographic printing or more particularly to a lithographic plate cleaner.
Lithography is a printing system taking advantage of the natural mutual repulsion between water and oils. The surface of a lithographic printing plate comprises areas which accept water and repel an oily ink, and areas which repel water and accept an oil ink. The former areas constitute non-image areas, and the latter areas constitute image areas.
Lithographic plates used in the graphic arts have image areas which are oleophilic and non-image areas which are hydrophobic and maintain the definition of the ink when the plate is used in the press to produce a clear image. After repeated use of the plate and aging of the surface, the non-image areas of the plate are less able to reject ink and may tend to retain some ink. This is called scumming.
Accordingly, if the surface properties between these two areas is disturbed, for example, if the hydrophilic property of the non-image areas is deteriorated for some reason, inks are apt to adhere to such hydrophilicity-deteriorated areas to cause background stains.
Such background stains are formed under various conditions. Typically, background stains are formed in the case where a lithographic printing plate is subjected to a burning-in treatment for the purpose of imparting high printing durability (i.e. run-length of printing) or in the case where the surface of a lithographic printing plate is allowed to stand in air without protection with a desensitizing gum.
The image areas of the plate may also become less oleophilic and hence less able to hold the ink. This may result in what is known as image blinding.
This results in poor quality printing and the lithographer must then extend a good deal of effort to restore the plate to a useful condition.
These phenomena sometimes occur even when operation of a printing machine is suspended, e.g because of disorders during printing or for recesses. Therefore, it is common to apply a desensitizing gum onto the printing plate during suspensions of printing. Further, when a lithographic printing plate unprotected with a desensitizing gum is left to stand with olephilic substances being attached to the non-image areas thereof, such contaminated areas are rendered oil-sensitive, thus resulting in formation of stains. For example, the appearance of fingerprints in the background of prints is ascribed to this phenomenon. Furthermore, when the non-image areas take scratches, the scratches are filled with an ink and are generally rendered oil-sensitive to cause stains.
In these cases, it is usual that a printing ink is first removed from the lithographic printing plate and the stained plate is then treated with a plate cleaner for restoring the hydrophilic property of non-image areas.
Such a plate cleaner conventionally includes a cleaner comprising an aqueous solution of sodium silicate. However, this plate cleaner, despite of its very high desensitizing activity, is disadvantageous in that a part of an image is damaged or adhesion of an ink to image areas is deteriorated because of its alkaline property when it is applied to lithographic printing plates.
Cleansing agents hitherto used consist of dispersions and contain a number of heterogeneous substances such as, for example, in the dispersion described in U.S. Pat. No. 2,780,168, which serves as cleansing agents for greasy printing plates and contains, in the aqueous phase, monofunctional and multifunctional alcohols and a colloidal dispersion of silicic acid and, in its non-aqueous phase, preferably petroleum ether.
Various other compositions have also been used heretofore as cleansing solutions. For example, U.S. Pat. No. 3,108,535 employs various phosphonic acids. U.S. Pat. No. 3,289,577 uses various sulfonic acids. U.S. Pat. No. 3,060,848 describes a plate cleaner having significant amounts of aromatic solvents. Likewise U.S. Pat. No. 4,162,928 employes a substantial amount of aromatic solvents. Plate cleaners are also described in U.S. Pat. Nos. 2,780,186 and 3,679,479.
Furthermore, a plate cleaner using oxalic acid, as disclosed in U.S. Pat. No. 3,489,561, has a weak desensitizing property and an activity to corrode a metal support. Therefore, when it is applied to commonly employed lithographic printing plates comprising an aluminum plate as a support, the hydrophilic layer provided on the support, for example, a layer formed by a surface treatment to render hydrophilic, as described in U.S. Pat. No. 2,714,066, is susceptible to destruction to cause stains. Therefore, such a plate cleaner is not adequate for metal supports.
In general, when stains generate during printing, the printing plate surface is first treated with an ink removing hydrocarbon type solvents to remove the ink, and then with a desensitizing agents. The above-described plate cleaner is also used as a desensitizing agents after the removal of ink. This means that the treatment for cleaning the plate surface should be done over two steps.
The present invention improves upon the aforementioned cleaners and provides a cleaner which corrects and prevents toning and scumming and does not detrimentally affect the image or non-image areas of the plate and yet is essentially free of aromatic hydrocarbons.